The present invention relates to a garnet-type magnetic material, a magnetic film with a high Faraday rotation incorporating such a material and the process for the production thereof.
More specifically it relates to the production of monocrystalline magnetic films which can be used in magneto optical devices, such as facsimile and display devices using the Faraday effect.
The principle of such devices is to obtain a brightness contrast using the Faraday effect induced on a monochromatic light on traversing a magnetic material. In such devices, use is made of a monocrystalline transparent substrate on which is deposited a ferrimagnetic garnet-type monocrystalline film in which the magnetization is normal to the plane, said film being subdivided by etching into magnetic elementary cells, whose magnetization can be oriented in one or other direction. These cells can consequently be displayed in polarized light as a result of the Faraday effect. Thus, cells oriented in one direction will appear plain, whereas the cells oriented in the other direction will appear dark. In order to use such devices for display purposes, it is consequently necessary to be able to reverse the magnetization direction in each of the elementary cells by appropriate means. Hitherto, two procedures have been proposed for achieving this result.
Accoding to a first procedure, the magnetization direction is reversed by a thermomagnetic effect using a localized heating pulse on the cell in question, as described by
B. Nill, K. P. Schmidt, "Fast switchable magneto-optic memory display contents, " Philips J. Res. 33 211 (1978); PA1 P. Hansen, B. Hill, W. Tolksdorf, "Optical switching with bismuth substituted iron garnets, " Philips Tech. Rev. Al, 33, 1984 PA1 and in European Pat. No. 00 23 063, filed on July 9, 1980 by Philips. PA1 G. R. Pulliam, W. E. Ross, B. MacNeal, R. F. Bailey, "Large stable magnetic domains", J.A.P. 53, 2754 (1982). PA1 L. Waller, "Compact displays--do it with magnetics", Electronics, Mar. 24, 1983, 51. PA1 M. F. Shone, V. R. K. Murphy, R. F. Belt, "Growth and magnetic properties of bismuth films for magneto-optic devices", IEEE (Trans on Mag) MAG 18 no. 6, pp. 1307-1309 (1982).
According to a second procedure, the result is achieved by a magnetic effect by the selective activation in the presence of a polarization field of thin conductors deposited on the magnetic film or layer in the form of two lattices, which are independent and perpendicular and which surround the elementary magnetic cells. The use of this procedure is in particular described in the following documents:
In each of these procedures, the magnetic material used for producing the film must have very precise characteristics, but the latter differ as a function of whether the thermomagnetic or magnetic effect is used.
Thus, in the case of the thermomagnetic effect, the inversion of the magnetization direction is obtained by the application of a polarization field associated with a heating pulse localized on certain cells. Therefore the material must have a compensation temperature close to ambient temperature in order that the action of the external field applied is zero on the unheated cells, which occurs when in the vicinity of the compensation temperature, where the resultant of the magnetizations of the sublattices of the garnet structure is cancelled out and brings about a zero action of an external field. However, the magnetic cells which would have been raised to a higher temperature will have their magnetization aligned in the direction of the simultaneously applied field and this will lead to the reversal of the magnetization direction.
Magnetic garnets liable to comply with these characteristics have the composition (GdBi).sub.3 (FeGaAl).sub.5 O.sub.12.
On using the second procedure for obtaining the reversal of the magnetization direction of the magnetic film or layer cells, the switching thereof takes place with the aid of currents circulating in crossed conductors in the presence of a polarization field. In this case, the magnetic material used must have characteristics differing widely from those of materials using the thermomagnetic effect. Thus, this material must not have a compensation temperature close to ambient temperature. However it must have a weak magnetization and a not very high anisotropy. Materials having these characteristics can comply with formula (BiTm).sub.3 (FeGa).sub.5 O.sub.12.
The use of the second procedure is particularly advantageous, because it makes it possible to obtain much more rapidly the reversal of the magnetization direction of the magnetic cells, which constitutes an important advantage, particularly in display means.